1. Field of the Invention
The present invention relates to a servo control apparatus, and specifically relates to a servo control apparatus having a function of obtaining a frequency response of a control system on line using the Fourier series.
2. Description of Related Art
A lot of apparatuses and methods related to the measurement of the frequency characteristics of a control system of a servo control apparatus have been proposed and widely used in measuring the transmission characteristics of a feed axis of a machine tool. Generally speaking, while a vibrator forcefully applies a vibration to an analysis object (while applying a frequency sweep), a response vibration is measured. After time series data thereof is logged into mass storage, a frequency transfer function is obtained using various signal processing techniques. In simplified measurements, an actuator itself is used as the vibrator, and a response vibration thereto is measured, logged, and subjected to signal processing. In electric circuits and optical systems, a vibrator circuit is used.
A technology for understanding the resonance characteristics of an object by logging and analyzing time series data has been actively developed (for example, “System Identification for Control by MATLAB”, written by Shuichi Adachi, published in 1996 by Tokyo Denki University Press, pp. 69-88, hereinafter referred to as “non-patent literature 1”). As described in the non-patent literature 1, the above method for calculating a frequency response belongs to “non-parametric identification” that is premised on the Fourier transform of the time series data. As a method for “parametric identification” referred to as online estimation (sequential estimation), there is a normalized gradient method, which is easily realized using a DSP (digital signal processor) (for example, “System Identification for Control by MATLAB”, written by Shuichi Adachi, published in 1996 by Tokyo Denki University Press, pp. 89-114 and pp. 115-151). These methods basically aim to obtain the frequency characteristics in the course of or as a result of fitting to a linear regression model.
A non-parametric measurement method of the frequency characteristics applied to a general closed loop control and a simplified measurement method that eliminates the need for providing a spectrum analyzer are known (for example, Japanese Unexamined Patent Publication (Kokai) No. 59-226907, hereinafter referred to as “patent literature 1”). As described in the patent literature 1, conventional measurement methods of the frequency characteristics use the Fourier transform by a spectrum analyzer.
There is also known an application in which the measurement of the frequency characteristics using the Fourier transform is used for a machine tool (for example, Japanese Patent No. 5302639, hereinafter referred to as “patent literature 2”). The patent literature 2 describes a machine tool control apparatus that regulates a compensation circuit based on the calculation of the frequency characteristics by a sinusoidal sweep method. The frequency characteristics are calculated before the machine tool is actually operated (refer to paragraph [0031] of patent literature 2).
There are proposed methods that do not use the Fourier transform (for example, Japanese Unexamined Patent Publication (Kokai) Nos. 2004-020522 and 11-326411, hereinafter referred to as “patent literature 3” and “patent literature 4”, respectively). In these methods, the time difference between input and output signals is directly measured to obtain a phase delay. The patent literature 3 discloses a measurement method for calculating the frequency characteristics by direct current detection of an output and zero-crossing detection when analyzing the output signal of a transmission path relative to a sinusoidal wave input, and a method for obtaining a phase using a time measurement module (TMS). The patent literature 4 discloses a frequency characteristics measurement device that calculates the frequency characteristics in a sequential manner by a sequence control. In frequency switching, waiting time is provided for data sampling, using measurement parameters including a group delay time and an allowance.
A method for obtaining a frequency transfer function by making complex Fourier coefficients into vectors is proposed (for example, Japanese Patent No. 3626858, hereinafter referred to as “patent literature 5”). This method is meaningful in terms of enabling analyses of a vibration containing harmonics. The patent literature 5 discloses that complex Fourier coefficients of an observation vibration containing harmonics are calculated at a vibration axis in a multi-axis vibration table and the transfer function of the vibration table is estimated from the vectors of the Fourier coefficients, and a vibration analysis method in a case where an actuator applies a vibration to a system in which an accelerometer is attached to the vibration table. The invention described in the patent literature 5 originally relates to the vibration table system on which a specimen is mounted. The vibration table aims at testing the strength of the specimen, and a waveform distortion control device is a vibration reproduction device. The vibration table system is not a control system for positioning using an electric motor.
The use of the methods requiring the logging of the time series data, as described in the patent literatures 1 and 2, allows precise obtainment of the frequency characteristics using the Fourier transform. The fast Fourier transform is realized by a method referred to as a butterfly computation. A spectrum analyzer such as an oscilloscope is equipped with an adequate volatile memory area and a butterfly circuit. This enables the Fourier transform in real time with ease. However, the control period of digital servo control greatly depends on the PWM control period of a servo amplifier, which is a driver of a motor. Thus, the servo control period is determined under an operation speed constraint of a power device installed in the amplifier. Due to heat generation by the power device, the servo control period cannot be so short. Due to heat generation by a control DSP itself, a clock speed of the DSP cannot be so high. For these reasons, it is difficult to realize the fast Fourier transform in real time by only software installed for the servo control.
The methods that do not use the Fourier transform, as described in the patent literatures 3 and 4, allow the calculation of the frequency characteristics online in view of an amount of calculation and memory consumption. However, the methods that “directly” perform the zero-crossing detection and measure the delay time of the time series data cannot calculate the frequency characteristics with high precision. In the machine feed axis, a nonlinear vibration (so-referred to as self-excited chatter vibration and the like) often becomes conspicuous. In such a system, the frequency characteristics different from the actual resonance characteristics are obtained, owing to prominent integral multiple harmonics.
The patent literature 5 proposes a method for precisely calculating the frequency characteristics in a nonlinear system having such harmonics. The Fourier series, which does not obtain a continuous spectrum but obtains a discrete spectrum, is suitable for the calculation of the frequency characteristics online, because of the easy realization with a limitation on an arbitrary number of terms. However, the proposal described in the patent literature 5 relates to a method for configuring the device to determine the strength of an object, but does not relate to the structure and control for positioning a machine tool. The patent literature 5 just proposes a compensation element to reproduce a time series signal, but proposes neither a method for calculating the frequency characteristics nor a measurement method for essential control characteristics.